This invention relates to thermal ink jet printers and more particularly to a thermal ink jet printer having a temperature sensor as a permanent part of the ink cartridge receiving carriage, so that the temperature of the printheads of replaceable ink cartridges may be continually monitored.
The ink droplet ejecting performance of thermal ink jet printheads is temperature dependent. As temperature increases, so does the volume of the ink ejected. Moreover, at sufficiently high temperatures, air ingestion causes unreliable droplet ejection, leading to missing ink spots on the recording medium. Methods have been devised to compensate for printhead temperature variations and the thermal effects created by the temperature variations. These methods include modifying the electrical pulses to the droplet ejecting heating elements in response to the temperature of the printhead to keep the droplet volume and thus spot size more constant regardless of printhead temperature and also to suppress air ingestion. The electrical pulses are generally modified by varying the electrical pulse width and/or amplitude. Another method to compensate for thermal effects on printheads includes applying heat to the printhead when it is too cold, such as, by applying subthreshold pulses to the heating elements which are not capable of ejecting droplets. Most of the methods for compensating for printhead thermal effects require an accurate measurement of printhead temperature. One known method of providing a temperature reading for a printhead is to fabricate a temperature sensor or measuring device, such as a thermistor, directly on the thermal ink jet printhead die. The fabrication is economical, but in some applications it may be difficult to have a known thermal response of the sensor without calibrating each part. Another method is to bond a temperature measuring device directly to the printhead. In some applications, such as, for example, in printers which use disposable printhead cartridges, it is not economical to bond a sensor to each printhead. It is more economical to incorporate a permanent temperature measuring device into the printer, so that it is not thrown away each time the cartridge with the printhead is disposed. Generally, such a temperature measuring device in the printer is only capable of measuring ambient printer temperature, because the temperature measuring device does not contact the printhead. There is significant self-heating in a thermal ink jet printhead, so that the printhead temperature is somewhat different than ambient temperature. Thus, temperature prediction algorithms must be used based on ambient temperature and the printhead usage rate, whenever the ambient printer temperature is used. However, it is not easy to make such algorithms quantitatively accurate for all usage and ambient conditions.
U.S. Pat. No. 4,899,180 to Elhatem, et al. discloses an ink jet printhead having integrated into it a number of heating elements and a temperature regulating circuit to heat the printhead to its optimum operating temperature within seconds of turn-on and thereafter maintain that temperature.
U.S. Pat. No. 4,980,702 to Kneezel et al. discloses a printhead bonded to a heat sinking substrate having a recess formed therein. Two layers of resistive material, separated from each other by a dielectric layer, are formed in the recess by a thick film screen print process which functions respectively as a heater and a temperature sensor. The recess underlies the printhead which is bonded to the substrate. The arrangement provides good proximity of the heater and the temperature sensor to the printhead, thereby enabling accurate temperature measurements and efficient printhead heating.
U.S. Pat. No. 5,075,276 to Kneezel discloses an ink jet printhead fabricated with a resistive temperature sensor formed adjacent the heating elements. In the preferred embodiment, the sensor and heating elements are the same material. The resistive value of the sensor is established by a trimming operation while the printhead is at a specified set temperature. This technique provides the accuracy of the sensor or thermistor required which was otherwise not possible when the thermistor was formed in close proximity to the printhead and of the same material as the heating elements.
U.S. Pat. No. 5,107,276 to Kneezel et al. discloses a thermal ink jet printer having a printhead maintained at a substantially constant operating temperature during printing. Printhead temperature fluctuations are prevented by selective heating of the heating elements not being used to eject droplets with energy pulses having insufficient magnitude to eject droplets.
U.S. Pat. No. 5,223,853 to Wysocki et al. discloses a method of controlling the spot sizes printed by a thermal ink jet printer. The temperature of the ink in the printhead is sensed and a combination of power level and time duration of the electrical input signal to the heating elements is selected by entering the sensed temperature of the ink into a predetermined function relating to the energy of the input signal to the corresponding resulting size of the spot on the copy sheet.